I'm a science journalist and author of "Distant Wanderers: the Search for Planets Beyond the Solar System" who writes about over-the-horizon technology, primarily astronomy and space science. I’m a former Hong Kong bureau chief for Aviation Week & Space Technology magazine and former Paris-based technology correspondent for the Financial Times newspaper who has reported from six continents. A 1998 winner in the Royal Aeronautical Society's Aerospace Journalist of the Year Awards (AJOYA), I’ve interviewed Nobel Prize winners and written about everything from potato blight to dark energy. Previously, I was a film and arts correspondent in New York and Europe, primarily for newspaper outlets like the International Herald Tribune, the Boston Globe and Canada's Globe & Mail. Recently, I've contributed to Scientific American.com, Nature News, Physics World, and Yale Environment 360.com. I'm a current contributor to Astronomy and Sky & Telescope and a correspondent for Renewable Energy World. Twitter @bdorminey

Life's 'Left-Handed' Amino Acids Still A Puzzle

Some astronomers think life’s handedness may have origins in radiation-rich star-forming regions like the Cat’s Paw Nebula (NGC 6334) --- shown here in the near infrared. NGC 6334, a vast region of star formation about 5500 light-years from Earth in the constellation of Scorpius, remains one of the most active nurseries of massive young stars in our galaxy. Credit: ESO/J. Emerson/VISTA; Acknowledgment: Cambridge Astronomical Survey Unit

Did life’s predilection for so-called “left-handed” amino acids evolve as a quirk of prebiotic chemistry here on earth? Or did biomolecules’ reliance on single “chirality” or handedness actually have more offworld origins?

These are still two of astrobiology’s toughest questions.

Some researchers have even suggested that left-handed amino acids — molecules that can’t be superimposed on their mirror images and that rotate polarized light in opposite directions — may be a pre-requisite for life’s evolution anywhere in the galaxy.

That’s, in part, because life uses only left-handed amino acids in the construction of proteins that lead to its basic RNA and DNA building blocks.

The origin of biological chirality is a question that has plagued biochemists since the 19th century when French microbiologist Louis Pasteur first separated tartrate salt crystals into their molecular mirror images.

“Life could have emerged from either left or right handed amino acids, but not both,” said Mindy Levine, an organic chemist at the University of Rhode Island, who wrote her 2008 doctoral thesis on the origin of biological chirality. “There’s nothing to prevent life from developing from right-handed amino acids, it just happened that on our planet the left-handed amino acids were what was available.”

Meteorites could have delivered left handed amino acids to earth, says Levine, then in turn used that left-handed chirality in developing life. But Levine doesn’t think left-handed amino acids are a biological prerequisite.

“The problem with “chirality” is not so much “How could it possibly have happened?” but more “Which of many possible routes actually led to chirality?” says astrobiologist Steven Benner, a distinguished fellow at the Foundation for Applied Molecular Evolution (FFAME) and The Westheimer Institute for Science and Technology, in Gainesville.

Even so, Donna Blackmond, a chemist at The Scripps Research Institute in La Jolla, California, and colleagues have successfully modeled four viable ways to get from a small chiral imbalance to where we are today. But she doesn’t pretend to solve the conundrum of what started the imbalance.

If the root of the imbalance is astrophysical, then life’s left handedness might stem from our our sun’s own stellar birth cluster.

“A supernova, of the sort that could have triggered the formation of our solar system, may also have caused a left-handed molecular preference,” I write in my book, Distant Wanderers. “Radiation from the newly formed neutron star at the core of the supernova may have caused the scattering of ultraviolet radiation in dust particles…, causing the selection of one hand over another.”

In this jumble of circularly-polarized radiation and dust, left-handed amino acids may have formed around our young star and made their way into meteorites which were then delivered to our young earth.

In recent observations of the Cat’s Paw Nebula (NGC 6334), an active star-forming region in Scorpius, some 5500 light years away, Jungmi Kwon, an astronomer with the National Astronomical Observatory of Japan (NAOJ) in Tokyo, along with a team of international colleagues, have noted high degrees of circularly-polarized light. Both in her doctoral thesis and a paper appearing The Astrophysical Journal Letters, Kwon and the team respectively report the detection of “high degrees of circular polarization” in nine planet-forming regions; including Orion, Taurus, and Chamaeleon.

“The universality of circular polarization in various star-forming regions implies that it may be a common feature,” said Kwon.

An example of chirality of amino acids; using hands to illustrate that the molecules are mirror images of each other. Credit: NASA Goddard/Office of the Chief Technologist

Some astrobiologists have used this argument over handedness as a way to judge if life is likely to be ubiquitous in the galaxy. If we found life on Mars used right-handed amino acids, that might indicate that life was common throughout the galaxy, since it could emerge from either left- or right-handed amino acids.

But Blackmond turns that argument on its head.

“Would life somewhere else have to be based on amino acids and sugars?” Blackmond asks. “Would it have to have the same sense of symmetry? I don’t know if it would be based on the same molecules as us.”

To date, the astrophysics seems to support the left-handed argument in that right-handed amino acids have yet to be found in meteorites. But that doesn’t mean that the source of life’s left-handed amino acids couldn’t have arisen locally.

Benner notes that minerals such as quartz are chiral. But he says transferring that chirality to pre-biotic compounds would be very difficult, since minerals do not dissolve in water and, if they do, they generally do not maintain their chirality.

“A conclusive answer is going to be hard to come by,” said Levine. “The best we can hope for is a workable hypothesis with enough supporting evidence to give us a hint of what happened so many years ago.”

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